Flow battery having electrodes with a plurality of different pore sizes and or different layers

ABSTRACT

A flow battery includes an electrode operable to be wet by a solution having a reversible redox couple reactant. In one embodiment, the electrode can have plurality of micro and macro pores, wherein the macro pores have a size at least one order of magnitude greater than a size of the micro pores. In another embodiment, the electrode includes a plurality of layers, wherein one of the plurality of layers has a plurality of macro pores, and wherein another one of the plurality of layers has a plurality of micro pores. In another embodiment, the electrode has a thickness less than approximately 2 mm. In still another embodiment, the electrode has a porous carbon layer, wherein the layer is formed of a plurality of particles bound together.

This application is a continuation of U.S. patent appln. Ser. No.13/084,156 filed Apr. 11, 2011.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to PCT Application No. PCT/US09/68681 filedon Dec. 18, 2009, U.S. patent application Ser. No. 13/022,285 filed onFeb. 7, 2011 and U.S. patent application Ser. No. 13/023,101 filed Feb.8, 2011, each of which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Technical Field

This disclosure relates generally to a flow battery and, moreparticularly, to a flow battery having electrodes with a plurality ofdifferent pore sizes and/or different layers.

2. Background Information

A typical flow battery system includes a stack of flow battery cells,each having an ion-exchange membrane disposed between negative andpositive electrodes. During operation, a catholyte solution flowsthrough the positive electrode, and an anolyte solution flows throughthe negative electrode. The catholyte and anolyte solutions eachelectrochemically react in a reversible reduction-oxidation (“redox”)reaction. Ionic species are transported across the ion-exchange membraneduring the reactions, and electrons are transported through an externalcircuit to complete the electrochemical reactions.

The negative and positive electrodes can be constructed from a carbonfelt material. Such a carbon felt material typically has a plurality ofinterstices of substantially uniform size that promote uniformdistribution of the electrolyte solution therethrough. Each electrodehas a relatively large thickness (e.g., greater than 3.2 millimeters(mm), ˜125 thousandths of an inch (mil)) sized to reduce pressure dropacross a length of the electrode, which length is substantiallyperpendicular to the thickness. Such a relatively large electrodethickness, however, can substantially increase resistance to ionicconduction across the thickness of the electrode. Electrodes withrelatively large thicknesses, therefore, can increase voltageinefficiency of the flow battery cell due to the increased resistance toionic conduction, especially when the flow battery cell is operated atrelatively high current densities such as greater than 100 milli amps(mA) per square centimeter (cm²) (˜645 mA per square inch (in²)).

There is a need in the art, therefore, for a flow battery cell that canoperate at relatively high current densities, without significantlyincreasing voltage inefficiency. Operating at relatively high currentdensities without excessive voltage losses can permit use of a smallerstack size and, therefore, a lower stack cost for a given power output.

SUMMARY OF THE DISCLOSURE

The present invention includes a flow battery having an electrode (alsoreferred to as an “electrode layer”) that is operable to be wet by asolution having a reversible redox couple reactant. According to oneaspect of the invention, the electrode has a plurality of micro andmacro pores, wherein the macro pores have a size at least one order ofmagnitude greater than a size of the micro pores. According to anotheraspect of the invention, the electrode has a plurality of layers. One ofthe plurality of layers has a plurality of macro pores, and another oneof the plurality of layers has a plurality of micro pores. According toanother aspect of the invention, the electrode has a thickness less thanapproximately 2 mm (˜78 mil). According to still another aspect of theinvention, the electrode has a porous carbon layer, wherein the layer isformed of a plurality of particles bound together.

The foregoing features and operation of the invention will become moreapparent in light of the following description and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of a flow batterysystem, which includes a plurality of flow battery cells arranged in astack.

FIG. 2 is a diagrammatic illustration of one embodiment of one of theflow battery cells in FIG. 1, and an enlargement of a section of anelectrode layer included in the flow battery cell.

FIG. 3 is a diagrammatic illustration of one embodiment of the stack offlow battery cells in FIG. 1.

FIG. 4 is a diagrammatic illustration of another embodiment of one ofthe flow battery cells in FIG. 1, and an enlargement of a section of anelectrode layer included in the flow battery cell.

FIG. 5 is a diagrammatic illustration of another embodiment of one ofthe flow battery cells in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a schematic diagram of a flow battery system 10 isshown. The flow battery system 10 is configured to selectively store anddischarge electrical energy. During operation, for example, the flowbattery system 10 can convert electrical energy generated by a renewableor non-renewable power system (not shown) into chemical energy, which isstored within a pair of first and second electrolyte solutions (e.g.,anolyte and catholyte solutions). The flow battery system 10 cansubsequently be operated to convert the stored chemical energy back intoelectrical energy. Examples of suitable first and second electrolytesolutions include, but are not limited to, vanadium/vanadium electrolytesolutions and bromine/polysulphide electrolyte solutions.

The flow battery system 10 includes a first electrolyte storage tank 12,a second electrolyte storage tank 14, a first electrolyte circuit loop16, a second electrolyte circuit loop 18, and at least one flow batterycell 20. In some embodiments, the flow battery system 10 can include aplurality of the flow battery cells 20 arranged and compressed into atleast one stack 21 between a pair of end plates 39, which cells 20 canbe operated to collectively store and produce electrical energy. Theflow battery system 10 further includes a control system (not shown)that includes a controller, a power converter/regulator, and first andsecond electrolyte solution flow regulators (e.g., valves, pumps, etc.),which control system is adapted to control the storage and discharge ofelectrical energy by the flow battery system.

Each of the first and second electrolyte storage tanks 12 and 14 isadapted to hold and store a respective one of the electrolyte solutions.

The first and second electrolyte circuit loops 16 and 18 each have asource conduit 22, 24 and a return conduit 26, 28, respectively.

Referring to FIG. 2, a diagrammatic illustration of one embodiment ofthe flow battery cell 20 is shown. The flow battery cell 20 includes afirst current collector 30, a second current collector 32, a firstliquid-porous electrode layer 34 (hereinafter “first electrode layer”),a second liquid-porous electrode layer 36 (hereinafter “second electrodelayer”), and an ion-exchange membrane layer 38.

The first and second current collectors 30 and 32 are each adapted totransfer electrons to and/or away from a respective one of the first orsecond electrode layers 34 and 36. Referring to FIG. 3, in someembodiments, one of the first and one of the second current collectors30 and 32 in the stack 22 can be configured together as a bipolar plate41. The bipolar plate 41 is adapted to be arranged between adjacent flowbattery cells 20 within the stack 21. The bipolar plate 41 extendsbetween a pair of current collector surfaces 43 and 45. A first one ofthe surfaces, 43, is operable to function as the first current collector30 for a first one of the cells. A second one of the surfaces, 45, isoperable to function as the second current collector 32 for a second oneof the cells. Referring again to FIG. 2, each current collector 30, 32can include a plurality of flow channels 40, 42 having, for example,straight, curved or serpentine geometries. Additional examples ofsuitable current collector and channel configurations are disclosed inPCT Application No. PCT/US09/68681, which is hereby incorporated byreference in its entirety.

The first and second electrode layers 34 and 36 are each adapted tooperate at a relatively high current density (e.g., greater than orequal to approximately 100 mA/cm², ˜645 mA/in²). Each electrode layer34, 36 has a first surface 44, 46, a second surface 48, 50, a first end52, 54, a second end 56, 58, a thickness 60, 62 and a length 64, 66,respectively. The thickness 60, 62 extends between the first surface 44,46 and the second surface 48, 50, respectively. In one embodiment, thethickness 60, 62 is less than approximately 3 mm (˜118 mil). In anotherembodiment, the thickness 60, 62 is less than approximately 2 mm (˜78mil).). The relatively small thickness 60, 62 of each electrode layer34, 36 significantly reduces resistive ionic transport losses throughthe electrode layer 34, 36, relative to an electrode layer having athickness greater than, for example, 3.2 mm (˜125 mil). The length 64,66 extends in a direction along an electrolyte solution flow path 68,70, between the first end 52, 54 and the second end 56, 58,respectively.

Referring to the enlarged section of the second electrode layer 36 shownin FIG. 2, each electrode layer 34, 36 has a plurality of macro andmicro pores 72 and 74 defining an electrode layer volumetric porosity.The term “volumetric porosity” refers herein to a ratio of a volume ofthe pores (also sometimes referred to as “interstices”) in a material toa volume of the layer material. In one embodiment, for example, themacro and micro pores 72 and 74 define a volumetric porosity of lessthan approximately 9:1, pore volume to material volume. In anotherembodiment, the macro and micro pores 72 and 74 define a volumetricporosity of less than approximately 7:3, pore volume to material volume.The volumetric porosity can change, however, when each electrode layer34, 36 is compressed between the ion-exchange membrane layer 38 and arespective one of the current collectors 30, 32 during assembly. Themacro and micro pores 72 and 74 can define, for example, a volumetricporosity of between approximately 4:6 and 7:3, pore volume to materialvolume, after the respective electrode layer has been compressed.

Substantially all of the macro pores 72 have a size that is at least oneorder of magnitude greater than (i.e., 10 ¹ times) a size ofsubstantially all of the micro pores 74. In another embodiment, the sizeof substantially all of the macro pores 72 is at least two orders ofmagnitude greater than (i.e., 10 ² times) the size of substantially allof the micro pores 74. The size of each macro pore 72 is selected toreduce pressure drop across the electrode layer 34, 36 and, therefore,facilitate flow of the electrolyte solutions through the respectiveelectrode layers 34 and 36. The size of each micro pore 74, on the otherhand, is selected to maintain an adequate electrode surface area forelectrochemical interactions between the electrode layers 34 and 36 andthe respective electrolyte solutions. In one embodiment, for example,substantially all of the macro pores 72 have a diameter greater than orequal to approximately 100 micrometers (μm) (˜3.9 mil), andsubstantially all of the micro pores 74 have a diameter less than orequal to approximately 1 μm (˜39 microinches (μ in)).

The macro and micro pores 72 and 74 are arranged in a pattern torespectively reduce pressure drop and ionic losses across the electrodelayer 34, 36. In the embodiment shown in FIG. 2, for example, the macroand micro pores 72 and 74 are respectively arranged in a plurality ofcolumns 65, 69 that extend along the electrolyte solution flow path 68,70 between the first end 52, 54 and the second end 56, 58 of eachelectrode layer 34, 36. The columns 65 of macro pores can each have asubstantially equal macro pore density (i.e., the quantity of macropores per unit volume of the layer) such that the macro pores 72 areuniformly distributed in each electrode layer 34, 36. Similarly, thecolumns 69 of micro pores can each have a substantially equal micro poredensity (i.e., the quantity of micro pores per unit volume of the layer)such that the micro pores 74 are also uniformly distributed in eachelectrode layer 34, 36. Alternatively, as shown in the embodiment inFIG. 4, the macro and/or micro pores 172 and 174 can be non-uniformlydistributed in each electrode layer 134, 136. The column 165 of macropores 172 adjacent to the first surface 144, 146 of each electrode layer134, 136, for example, can have a higher macro pore density than thecolumns 167 of macro pores proximate the second surface 148, 150 inorder to reduce pressure drop adjacent the first surface 144, 146. Thecolumn 169 of micro pores 174 adjacent the second surface 148, 150 ofeach electrode layer 134, 136, on the other hand, can have a highermicro pore density than the columns 171 of micro pores proximate thefirst surface 144, 146 in order to increase (e.g., maximize) electrodesurface area for electrochemical interactions with the respectiveelectrolyte solutions in these regions. Increasing the electrode surfacearea can increase the density of the redox reactions that occur near thesecond surface 148, 150 of each electrode layer 134, 136 and therebyreduce ionic losses in the cell 120.

Referring again to FIG. 2, the macro and micro pores 72 and 74 can bearranged in the aforesaid patterns, for example, by forming eachelectrode layer 34, 36 from a plurality of electrode sub-layers 73 and75. A first one of the electrode sub-layers 73, for example, can beformed to include a plurality of the macro pores 72. A second one of theelectrode sub-layers 75, on the other hand, can be formed to include aplurality of the micro pores 74.

Each electrode layer 34, 36 can be constructed from a mixture of a solidelectronic conductor, a solvent, a pore former and a binder. The solidelectronic conductor can include mixtures of metals, graphite or carbonparticles, phenolic resin powder, cellulose fiber and/or carbon orgraphite fibers. The graphite or carbon particles powder can include atleast one of carbon fibers and spherical carbon particles. In oneembodiment, for example, a carbon power such as Vulcan XC-72(manufactured by Cabot Corporation of Boston, Mass., United States), asolvent such as isopropanol, a pore former such as ammonium carbonate orpolystyrene, and a binder such as a polymer or a resin are mixedtogether into an electrode ink. If the binder is hydrophobic, then itcan be treated in a subsequent process to make it more hydrophilic, oralternatively, the binder can be subsequently heat treated to removesubstantially all of the binder; e.g., carbonization can be utilized toform a carbon-carbon composite layer. Alternatively, a hydrophilicbinder can be included in the electrode ink, which can include anion-exchange polymer, or ionomer, such as a sulfonatedtetrafluoroethylene based fluoropolymer-copolymer or perfluorosulfonicacid (PFSA) (e.g., Nafion® polymer manufactured by DuPont of Wilmington,Delaware, United States). The ionomer (e.g., PFSA or any other type ofion-exchange polymer) is used as a binder and a supporting electrolytefor transport of ionic species in the electrode layer. The ionomer canbe mixed within the electrode ink such that it is uniformly ornon-uniformly distributed along the length of the electrode layer. Theelectrode ink mixture can also include an electrochemical catalyst. Theelectrochemical catalyst is selected to promote certainreduction-oxidation (“redox”) reactions in the electrolyte solutions.Examples of such an electrochemical catalyst include metals that can besupported or unsupported on a conductive support, such as carbon, toenhance the surface area of the catalyst. Examples of supported metalelectrochemical catalysts include a nickel catalyst dispersed on carbon(Ni/C), and a platinum catalyst dispersed on carbon (Pt/C). Theelectrochemical catalysts can be mixed within the electrode ink suchthat it is uniformly or non-uniformly distributed along the length ofthe electrode layer.

An electrode sub-layer structure for promoting reactant distribution canbe formed by adjusting a by-weight ratio of the pore former to the otheringredients by itself, or in combination with adjusting the size of thecarbon powder particles. The electrode sub-layer can be formed, forexample, with a relatively high porosity and, therefore, a relativelylarge number of macro or micro pores by including a relatively highby-weight ratio of the pore former in the electrode ink. The electrodeink is applied (e.g., sprayed or printed) onto a desired surface (e.g.,a surface of the ion-exchange membrane layer or a decal) to form one ofthe electrode sub-layers 73 or 75. Additional sub-layers can be formedon or attached to the previously formed sub-layer to construct one ofthe electrode layers 34 or 36. The pore former can be removed (e.g.,dissolved) from the electrode layer, for example, using a solvent wash(e.g., a hydrofluoric acid). In embodiments where the electrode ink doesnot include hydrophilic material (e.g., Nafion® polymer manufactured byDuPont of Wilmington, Del., United States), each electrode sub-layer orlayer can be subjected to additional treatments (e.g., chemical orelectrochemical oxidation) to provide hydrophilic properties. Heattreatment processes can also be used to convert carbon species to moregraphitic forms, which have improved corrosion resistance.

Referring to FIG. 5, in some embodiments, each electrode layer 34, 36 iscoupled to a backing layer 76, 78. The backing layer 76, 78 is adaptedto (i) provide mechanical support to the electrode layer 34, 36, (ii)improve uniform distribution of the electrolyte solution to theelectrode layer 34, 36, and (iii) provide a temporary electrolytesolution reservoir and/or additional surface area to complete the redoxreactions during rapid start up and transient conditions. The backinglayer 76, 78 can be constructed from a hydrophilic porous carbon layersuch as, but not limited to, a layer made from Toray ® fibers(manufactured by Toray Industries of Tokyo, Japan), carbon papers,carbon felts, or carbon cloths. In aforesaid embodiment, the electrodelayers 34, 36 can be formed onto the backing layers.

In other embodiments (not shown), each electrode layer 34, 36 includesat least one hydrophilic porous carbon layer having relatively large anduniform pore sizes. Examples of such a hydrophilic porous carbon layerinclude carbon papers, carbon felts, or carbon cloths. The hydrophilicporous carbon layer is impregnated with relatively small particles toform an electrode layer with bi-modal pore sizes; i.e., an electrodelayer with macro and micro pores. Inks of the particles, solvents, andbinders may be used to impregnate and bond the particles throughout theeach electrode layer. By constructing the electrode layers with theaforesaid technique, each electrode layer can be constructed withsubstantially uniformly distributed macro and micro pores. The aforesaidimpregnated electrode layer can also be combined with a backing layer asshown in FIG. 5.

Referring again to FIG. 2, the ion-exchange membrane layer 38 isconfigured to be permeable to certain non-redox couple reactants (alsoreferred to as “charge transport ions” or “charge carrier ions”) suchas, for example, protons (or H⁺ions) of vanadium electrolyte solutionsin acid. The ion-exchange membrane layer 38 is also configured to besubstantially impermeable to certain redox couple reactants (alsoreferred to as “non-charge transport ions” or “non-charge carrier ions”)such as, for example, V^(4+/5+)ions or V^(2+/3+)ions of the vanadiumelectrolyte solutions in acid. Examples of other redox couple reactantsinclude, but are not limited to, Fe^(2+/3+), Cr²⁺³⁺, Br−/Br₃ ⁻, S₂ ²⁻/S₄²⁻in acidic or basic solutions.

The ion-exchange membrane layer 38 is disposed between the first andsecond electrode layers 34 and 36. In one embodiment, for example, thefirst and second electrode layers 34 and 36 are formed on theion-exchange membrane layer 38, or the first and second electrode layers34 and 36 are hot pressed (e.g., from decals) onto opposite sides of theion-exchange membrane layer 38 to attach and increase surface interfacebetween the aforesaid layers 34, 36 and 38. In another embodiment, thefirst and second electrode layers 34 and 36 are bonded onto oppositesides of the ion-exchange membrane layer 38 with, for example, theaforementioned ionomer, which can also increase the interfacial surfacearea between the membrane and the electrode layer. The first and secondelectrode layers 34 and 36 are disposed between the first and secondcurrent collectors 30 and 32. Referring to FIG. 5, each electrode layer34, 36 can be connected to the respective current collector 30, 32 by arespective backing layer 76, 78. Referring again to FIG. 2, eachelectrode layer 34, 36 can be compressed between the ion-exchangemembrane layer 38 and a respective one of the current collectors 30, 32such that its thickness 60, 62 is reduced by at least 40% to furtherincrease interfacial surface area between the layers.

Referring to FIGS. 1 and 2, the flow battery cells 20, as indicatedabove, are arranged and compressed into a stack 21 between the pair ofend plates 39. The source conduit 22 of the first electrolyte circuitloop 16 fluidly connects the first electrolyte storage tank 12 to one orboth of the first current collector 30 and the first electrode layer 34of each flow battery cell 20. The return conduit 26 of the firstelectrolyte circuit loop 16 reciprocally fluidly connects the firstcurrent collector 30 and/or the first electrode layer 34 of each flowbattery cell 20 to the first electrolyte storage tank 12. The sourceconduit 24 of the second electrolyte circuit loop 18 fluidly connectsthe second electrolyte storage tank 14 to one or both of the secondcurrent collector 32 and the second electrode layer 36 of each flowbattery cell 20. The return conduit 28 of the second electrolyte circuitloop 18 reciprocally fluidly connects the second current collector 32and/or the second electrode layer 36 of each flow battery cell 20 to thesecond electrolyte storage tank 14.

Referring still to FIGS. 1 and 2, during operation of the flow batterysystem 10, the first electrolyte solution is circulated (e.g., via apump) between the first electrolyte storage tank 12 and the flow batterycells 20 through the first electrolyte circuit loop 16. Moreparticularly, the first electrolyte solution is directed through thesource conduit 22 of the first electrolyte circuit loop 16 to the firstcurrent collector 30 of each flow battery cell 20. The first electrolytesolution flows through the channels 40 in the first current collector30, and peimeates into and out of the first electrode layer 34 throughthe macro and micro pores 72 and 74; i.e., wetting the first electrodelayer 34. The permeation of the first electrolyte solution through thefirst electrode layer 34 can result from diffusion or forced convection,such as disclosed in PCT Application No. PCT/US09/68681, which canfacilitate relatively high reaction rates for operation at relativelyhigh current densities. The return conduit 26 of the first electrolytecircuit loop 16 directs the first electrolyte solution from the firstcurrent collector 30 of each flow battery cell 20 back to the firstelectrolyte storage tank 12.

The second electrolyte solution is circulated (e.g., via a pump) betweenthe second electrolyte storage tank 14 and the flow battery cells 20through the second electrolyte circuit loop 18. More particularly, thesecond electrolyte solution is directed through the source conduit 24 ofthe second electrolyte circuit loop 18 to the second current collector32 of each flow battery cell 20. The second electrolyte solution flowsthrough the channels 42 in the second current collector 32, andpermeates into and out of the second electrode layer 36 through themacro and micro pores 72 and 74; i.e., wetting the second electrodelayer 36. As indicated above, the permeation of the second electrolytesolution through the second electrode layer 36 can result from diffusionor forced convection, such as disclosed in PCT Application No.PCT/US09/68681, which can facilitate relatively high reaction rates foroperation at relatively high current densities. The return conduit 28 ofthe second electrolyte circuit loop 18 directs the second electrolytesolution from the second current collector 32 of each flow battery cell20 back to the second electrolyte storage tank 14.

During an energy storage mode of operation, electrical energy is inputinto each flow battery cell 20 through the current collectors 30 and 32.The electrical energy is converted to chemical energy throughelectrochemical reactions in the first and second electrolyte solutions,and the transfer of non-redox couple reactants from, for example, thefirst electrolyte solution to the second electrolyte solution across theion-exchange membrane layer 38. The chemical energy is then stored inthe electrolyte solutions, which are respectively stored in the firstand second electrolyte storage tanks 12 and 14. During an energydischarge mode of operation, on the other hand, the chemical energystored in the electrolyte solutions is converted back to electricalenergy through reverse electrochemical reactions in the first and secondelectrolyte solutions, and the transfer of the non-redox couplereactants from, for example, the second electrolyte solution to thefirst electrolyte solution across the ion-exchange membrane layer 38.The electrical energy regenerated by the flow battery cell 20 passes outof the cell through the current collectors 30 and 32.

While various embodiments of the present invention have been disclosed,it will be apparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. Each electrode layer, for example, can include one or moreadditional types of pores other than the aforedescribed macro and micropores. Accordingly, the present invention is not to be restricted exceptin light of the attached claims and their equivalents.

What is claimed is:
 1. A flow battery, comprising: an electrode layerincluding at least one macro pore sub-layer substantially defined bymacro pores, and at least one micro pore sub-layer substantially definedby micro pores, wherein the macro pores have a size at least one orderof magnitude greater than a size of the micro pores; wherein the macropore sub-layer and micro pore sub-layer each defines a solution flowpath through the electrode layer; and wherein the solution flow paththrough the macro pore sub-layer is configured to provide a firstpressure drop for a solution flow, and the solution flow path throughthe micro pore sub-layer is configured to provide a second pressure dropfor the solution flow, and the first pressure drop is less than thesecond pressure drop.
 2. The flow battery of claim 1, wherein the macropore sub-layer is independent of and contiguous with the micro poresub-layer.
 3. The flow battery of claim 1, wherein the sub-layers extendlengthwise between a first end of the electrode layer and a second endof the electrode layer.
 4. The flow battery of claim 1, furthercomprising: a current collector adapted to transfer electrons to andaway from the electrode layer; and an ion-exchange membrane layerdisposed between the electrode layer and a second electrode layer. 5.The flow battery of claim 4 further comprising a solution, whichsolution includes a reversible redox couple reactant.
 6. The flowbattery of claim 1, wherein the electrode layer is defined by athickness that extends between a first surface and a second surface. 7.The flow battery of claim 6, wherein the at least one macro poresub-layer includes a first macro pore sub-layer and a second macro poresub-layer.
 8. The flow battery of claim 7, wherein the first macro poresub-layer has a first density of macro pores and the second macro poresub-layer has a second density of macro pores, wherein the first andsecond densities are different.
 9. The flow battery of claim 8, whereinthe first surface is adjacent a current collector and the second surfaceis adjacent an ion-exchange membrane layer, and the first macro poresub-layer is disposed between the second macro pore sub-layer and thefirst surface.
 10. The flow battery of claim 6, wherein the at least onemicro pore sub-layer includes a first micro pore sub-layer and a secondmicro pore sub-layer.
 11. The flow battery of claim 10, wherein thefirst micro pore sub-layer has a first density of micro pores and thesecond micro pore sub-layer has a second density of micro pores, whereinthe first and second densities are different.
 12. The flow battery ofclaim 11, wherein the first surface is adjacent a current collector andthe second surface is adjacent an ion-exchange membrane layer, and thefirst micro pore sub-layer is disposed between the second micro poresub-layer and the second surface.
 13. The flow battery of claim 1,wherein the electrode later has a thickness of less than 2 millimeters.14. The flow battery of claim 1, wherein at least one of the macro poresub-layer and the micro pore sub-layer has a volumetric porosity of lessthan 9:1, pore volume to material volume.
 15. The flow battery of claim1, wherein at least one of the macro pore sub-layer and the micro poresub-layer has a volumetric porosity of less than 7:3, pore volume tomaterial volume.
 16. The flow battery of claim 15, wherein the macropores have a size at least two orders of magnitude greater than a sizeof the micro pores.